The invention relates to a method for determining the frequency for demodulation of received symbols or signal components in the complex phase space of a quadrature modulation method.
Correct determination of the frequency, i.e., the carrier frequency, is an important factor for a synchronization of a receiver or a receiving circuit arrangement for the reception of digital signals, coupled with a quadrature signal pair. The symbols represent, in encoded form, a one-place or multiple-place digital value. The encoding is done for transmission via the quadrature signal pair, which corresponds to a pointer, which assumes discrete positions in the Cartesian amplitude and phase space of the quadrature signal pair at certain moments in time. Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK) are transmission methods of this kind.
In a traditional receiver for reception of digital signals, a complex multiplier or mixer, which is triggered by a local oscillator, mixes the received QAM signal, modulated onto a carrier, in correct phase and frequency, into the base band of the circuit arrangement. The circuit arrangement usually has a phase-locked loop for the control process. In digital processing, this can come before or after an analog-to-digital (A/D) conversion. The signal is either sampled and digitized with the symbol clock cycle or a multiple thereof, or the digitization clock cycle is free-running relative to the required symbol clock cycle. In this case, the signal is converted via a purely digital sampling rate conversion to the symbol clock cycle or a multiple thereof. Gain controls make sure that the particular modulation range is utilized and that the received signals are correctly mapped onto the symbol decision-making stage. An adaptive equalizer reduces the intersymbol interference which is the result of linear distortions of the transmitter, the transmission pathway, or the receiver.
In high-order demodulators for QAM or PSK signals, the automatic control circuits for the frequency and phase control of the local oscillator, for gain control, for recovery of the symbol clock cycle, and for the adaptive equalizer require both the received symbols and the elements of the predetermined symbol alphabet that are considered by a decision-making stage as being the most probable. This type of control via the signal decided upon is known as “decision-feedback” control.
Since the decision-feedback controls are coupled together in the digital demodulators in the prior art, the locking process is difficult whenever the control for the local oscillator, which mixes the reception signal into the base band, is not yet stable in frequency and phase. Often, locking is only successful when the particular frequencies and phases are situated relatively close to their nominal values.
Demodulators for QAM or PSK signals ordinarily use a phase control which compares the received and sampled complex signal values to coordinates in the signal space that are assigned to symbols. Most often, one uses equidistant decision-making thresholds in both the I and Q direction of the complex signal space. A received phase point is coordinated with the nominal point of a symbol that represents the midpoint of an I/Q decision square lying in the complex I/Q plane.
A method which uses fields with radii and sectors, instead of the quadratic decision-making fields, is known from DE 36 19 744. In EP 0 281 652, groups of closely adjacent radii are first determined, and then in a following step the most suitable phase angle is determined on one of the radii in question. Decision-making devices with a limited symbol selection (reduced constellation) are used in U.S. Pat. No. 5,471,508 to avoid wrong rotations in the higher-order types of modulation.
In the known methods, the phase capture region is very small, especially for higher-order modulations. But until such time as the carrier phase control of the circuit arrangement is locked in, the symbols decided upon are often not correct, and as a result a wrong direction of rotation will be calculated for certain symbols. If the sum signal of all the correction signals is plotted against the phase deviation, unwanted zero points are obtained in the higher-order modulation methods, which result in a faulty lock-in.
Various methods are known for enlarging the capture range and avoiding false zero points for the sum correction signal. For example, in U.S. Pat. No. 5,471,508; EP 0571788; DE 36 19 744; DE 41 00 099; DE 44 10 607 and DE 199 28 206, one always starts with fixed nominal coordinates of the symbols in the signal space. The phase capture range cannot be expanded without special logic measures, for example according to EP 0571788.
All procedures for increasing the phase control range, however, result only in slight improvements in the frequent need to correct a frequency difference between the reception signal and the local oscillator and do not fundamentally solve the problem of a need for a frequency control method. Such a frequency difference signifies a rotation of the coordinate system of the input signal relative to the coordinate system of the circuit arrangement with constantly changing phase offset.
If a frequency difference has to be corrected, the loop gain of the carrier controls needs to be set so high that the phase, after correctly passing through the zero point, where no phase difference exists between the input signal and the local oscillator, is captured and held inside the small range in which the decision maker furnishes only correct or mostly correct decisions, (i.e., the local oscillator must be placed at the correct frequency and phase). However, the necessary stability of the automatic control loop restricts the possible loop gain.
Therefore, there is a need for a system and method for determining the frequency of a received signal for the demodulation of received symbols or signal components in the complex signal space of a modulation method with a better mode of operation.